1. Field of Invention
This invention relates to porous fabrics that may be used as filters, membrane supports, or more specifically, as battery separator fabrics.
2. Prior Art
One of the most difficult problems that has been encountered today in the battery separator art, is to economically provide a porous fabric for use as a battery separator. Although the prior art has developed a microporous film that is used as a battery separator, it has not produced a porous fabric or a film that is economical to use. The present invention has succeeded in providing a porous fabric that is inexpensive to produce where the prior art has not.
Typical prior art porous fabric is such as that made by the Celanese Corporation, located in Charlotte, N.C., under the trade name "Celgard." This prior art film is an extruded plastic film that is drawn out to make it porous. Drawing out is defined as a stretching process. Drawing of the film is done so as to make fractures occur in the film. There are disadvantages with this prior art. One such disadvantage is that it is very expensive to produce the porous fabric due to the number of process steps. Because the porous fabric is expensive to produce, the selling costs are high. Another disadvantage of this prior art is that the porous film is over engineered for use as a battery separator. Micropores formed in the fabric are too small in size for the film to be economically used as a battery separator. The pores in the prior art range between 0.02 and 0.04 microns. This is a range that is far below what is necessary in a battery separator. The allowable range of pore sizes in battery separator fabrics is preferably between 1 to 12 microns, with an average of 6 microns.
The present invention provides sufficient flow and sufficient barrier properties to serve as well or better than the prior art and is less expensive to produce.
In U.S. Pat. No. 4,472,328, there is described a porous film or sheet. The prior art porous film is made by melt-molding a resin selected from a group consisting of a linear low density polyethylene, a high density polyethylene and a polypropylene into a film or sheet. The film or sheet is then drawn out or stretched. During the stretching of the film, the film fractures, thus producing pores within the film or sheet.
This prior art has the same disadvantages as the previously mentioned prior art.
The present invention is superior to the prior art by reason of its economy, and more realistically fits the requirements of a battery separator material.
An object of the present invention is to provide a porous fabric that is more economical to produce than prior art.
Another object of the present invention is to provide a porous fabric that may be used as a battery separator.